Molecular beam epitaxy of Si1-xGex alloys, grown on (100)-oriented Si substrates by using gaseous Si2H6 and GeH4 as sources in the alloy composition range x < 0.33, and on (100)-oriented and (100)-misoriented Si substrates by using gaseous Si2H6 and solid Ge as sources over the alloy composition range (0 less than or equal to x < 1), has been studied. From the observed growth rates as a function of composition it is evident that the presence of Ge tends to decrease the Si incorporation rate. Our results indicate growth via adatom migration to kink sites in a dissociative chemisorption process.